6-Mirror projection objective with few lenses

ABSTRACT

There is provided a projection objective for wavelengths of ≦193 nm for imaging an object field in an object plane into an image field, in an image plane. The projection objective includes a first reflective optical element, a second reflective optical element, a third reflective optical element, a fourth reflective optical element, a fifth reflective optical element, and a sixth reflective optical element, and a refractive optical element. The reflective and refractive optical elements each have an off axis segment with a diameter. The projection objective has an image-side numerical aperture ≧0.65, and the off axis segment of the refractive optical element has a diameter that is less than ⅓ rd  of the distance from the object plane to the image plane.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is claiming priority of U.S. ProvisionalPatent Application Serial No. 60/368,180, filed on Mar. 28, 2002.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a projection objective for shortwavelengths, preferably of ≦193 nm, especially ≦157 nm, and also to aprojection exposure apparatus and a chip production, each of whichemploys such a projection objective.

[0004] 2. Description of the Related Art

[0005] Lithography with wavelengths of <193 nm is discussed as apossible technique for imaging structures of <130 nm and with specialpreference <100 nm. The resolution of a lithographic system is describedby the following equation:

RES=k ₁·(λ/NA)

[0006] wherein k₁ is a specific parameter of the lithographic process, λis the wavelength of the incident light, and NA is the numericalaperture of the system on the image side.

[0007] For imaging systems in the wavelength region of <193 nm,essentially reflective components are available in addition torefractive components made of fluorides.

[0008] Purely reflective systems with six mirrors for microlithographywith wavelengths of <193 nm have been disclosed by the publications U.S.Pat. No. 5,686,728 and EP 0 779,528, and U.S. Pat. No. 5,815,310.

[0009] The projection lithography system according to U.S. Pat. No.5,686,728 shows a projection objective with six mirrors, with each ofthe reflective mirror surfaces being of aspherical design. The mirrorsare arranged along a common optical axis in such a way that anobstruction-free light path is achieved.

[0010] To reduce the residual errors of the objective and to achievelarger apertures at the outlet pupil, EP-A-0 779,528 suggestsintroducing three refractive optical elements into the beam path. Adrawback of this embodiment with few lenses is, that the image-sidenumerical aperture NA is equal to 0.6, and the refractive opticalelements have large diameters, which is undesirable with respect toavailability of materials and costs, as well as manufacturingfeasibility. It is another drawback that the refractive optical elementsare off-axis segments, which are difficult to mount and to align.

[0011] An objective that is similar to that from EP-A-0 778,528 isdescribed in U.S. Pat. No. 4,701,035. FIG. 12 of U.S. Pat. No.4,701,035, for example, shows an objective with nine mirrors, twolenses, and two intermediate images. The objective that is described inU.S. Pat. No. 4,701,035 has essentially the same drawbacks as the systemknown from EP-A-0 779,528, i.e. a small image-side numerical aperture.Furthermore, the optical elements, as in EP-A-0 778,528, are difficultto mount and to align.

[0012] Catadioptric projection objectives have been disclosed byEP-A-1,069,448 and WO-A-01/51979 with an image-side numerical apertureof NA>0.6. The objectives disclosed by both EP-A-1,069,448 andWO-A-01/51979 are systems centered around an optical axis, with thenumber of refractive elements always being greater than that ofreflective elements. A drawback to these systems is their use of muchlens material.

SUMMARY OF THE INVENTION

[0013] It is thus the object of this invention to describe a projectionobjective device suitable for lithography at short wavelengths that doesnot have the drawbacks of the prior art mentioned above; particularlyone that makes available a high image-side numerical aperture withminimal use of lens material.

[0014] In addition to this, the optical elements should be readilymounted and easy to align.

[0015] According to the invention, the object is solved in a firstembodiment of the invention by the fact that in a projection objectivewith six reflective optical elements and at least one refractive opticalelement, the image-side numerical aperture NA is ≧0.65 and therefractive optical elements have a used diameter D of the off axissegments, the so called used area, of a optical element—that is smallerthan ⅓^(rd) of the distance of the object plane from the image plane,preferably less than ¼^(th), and with special preference less than⅕^(th) of the distance of the object plane from the image plane. In thepresent application, the used diameter D of the off-axis segment meansthe diameter of the circular envelope that encloses the off axissegment, the so called used area of a optical element, on the particularreflective or refractive component. The circular envelope is always thesmallest circle that encloses the off-axis segment of a optical element.The off axis segment of a optical element is also denoted as opticalfootprint.

[0016] In an especially preferred embodiment of the invention, theobjective is of very compact design with a total structural length ofless than 700 mm, preferably 500 mm. The refractive optical elementshave a diameter D of the off axis segment of the optical element that issmaller than 120 mm. Such refractive optical elements can be madesubstantially lighter than refractive elements with large lensdiameters, such as those disclosed by EP-A-0 779,528, for example.

[0017] In particular, the material needs for lens material and themachinability of the lens material, for example, CaF₂, is simpler withsmall diameters of the off axis segments of refractive components.

[0018] In an especially preferred embodiment of the invention, theprojection objective comprises three refractive optical elements, withtwo of the three refractive optical elements being positioned in thebeam path from the object plane to the image plane, after the sixthreflective element. The surface of the refractive optical element thatis closest to the image plane in the beam path is preferably ofaspherical design.

[0019] In an especially preferred embodiment, it is shown thatreflective elements in a definite portion of the off axis segment canalso be used simultaneously as refractive optical elements, i.e. Manginmirrors.

[0020] For technological reasons it is especially preferred for onlyfive of the six reflective optical components to be of asphericaldesign, and for the reflective optical component whose off axis segmenthas the greatest distance from the optical axis of the projectionobjective to be of spherical design. The off axis segment of theprojection objective denotes the area of a mirror that is illuminated onthe mirror when the object plane of the projection objective isilluminated with a segment of an annular field. The distance of the offaxis segment to the optical axis is the radial distance of the centerpoint of the off axis segment to the optical axis HA of the projectionobjective. The center point of the off axis segment is given by thecenter point of the annular field that illuminates the off axis segmentof a mirror.

[0021] In an especially preferred embodiment of the invention, thephysical diaphragm is positioned on or near the second reflectiveoptical component in the beam path from the object plane to the imageplane. In alternative configurations, the physical diaphragm can bepositioned between the first and second reflective optical components orbetween the second and third reflective optical components in the beampath from the object plane to the image plane.

[0022] In another embodiment of the invention, an intermediate image Zis formed in the beam path from the object plane to the image plane, ina plane conjugated to the object plane. The intermediate image ispreferably formed in the beam path from the object plane to the imageplane after the fourth reflective optical component. The intermediateimage divides the projection system into two sub-objectives, a firstsub-objective comprising first optical elements that are positioned inthe beam path from the object plane to a plane conjugated to the objectplane, and a second sub-objective comprising second optical elementsthat are positioned in the beam path from the conjugated plane to theimage plane. According to the invention, the first and secondsub-objectives each comprise no more than three refractive opticalelements. It is especially preferred for the first sub-objective to bedesigned so that the object in the object plane is imaged in the realintermediate image in the plane conjugated to the object plane with areduction ratio β that is approximately |β|=1, and the secondsub-objective images the real intermediate image in an image of theobject in the image plane with a reduction ratio |β|<1, preferably|β|<0.25.

[0023] Besides the projection objective, the invention also makesavailable a projection exposure apparatus, wherein the projectionexposure apparatus comprises a lighting device for lighting a field,preferably an annular field, and a projection objective according to theinvention. The invention will be described below with reference to theexamples of embodiment by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The figures show:

[0025]FIG. 1: the off axis segment of a reflective or refractiveelement,

[0026]FIG. 2: the annular field in the object plane of the objective,

[0027]FIG. 3: an embodiment of a projection objective according to theinvention with six reflective components and three refractivecomponents.

DESCRIPTION OF THE INVENTION

[0028]FIG. 1 illustrates what is meant in the present application by theoff axis segment and the diameter of the off axis segment.

[0029]FIG. 1 by way of example shows a kidney-shaped field for akidney-shaped illuminated field 1 on a reflective or refractive elementof the projection objective. Such a shape for the off axis segment isexpected when using the objective according to the invention in amicrolithography projection exposure apparatus when the object plane isilluminated with a segment of an annular field. The circular envelope102 fully encloses the kidney shape and coincides at two points 106,108, with the edge 110 of the kidney shape. The circular envelope isalways the smallest circle that encloses the off axis segment. Thediameter D of the off axis segment is then found from the diameter ofthe circular envelope 102.

[0030]FIG. 2 illustrates the object field 111 of a projection exposureapparatus in the object plane of the projection objective that is imagedusing the projection objective according to the invention in an imageplane in which a light-sensitive object is positioned, for example awafer. The image field in the image plane itself has the same shape asthe object field, but reduced in size by the reduction ratio. The objectfield or image field 111 has the configuration of a segment of anannular field. The segment has an axis of symmetry 112.

[0031] Also shown in FIG. 2 are the axes defining the object plane,namely the x-axis and the y-axis. As shown in FIG. 2, the axis ofsymmetry 112 of the annular field 111 runs in the direction of they-axis. Furthermore, the y-axis coincides with the scanning direction ofa projection exposure apparatus, which is designed as a scanner. Thex-direction is then the direction that is perpendicular to the scanningdirection within the object plane. The annular field has a so-calledcentral annular-field radius R that is defined by the distance of thecenter point 115 of the image field to the optical axis HA of theprojection objective.

[0032]FIG. 3 illustrates an embodiment of an objective according to theinvention. The same reference symbols are used for the same structuralelements as in FIGS. 1 and 2.

[0033] The annular field 111.1 in the object plane 120 of themicrolithography projection objective is imaged by the objective in animage field 111.2 with the same geometry, but reduced in size by thereduction ratio, in the image plane 122 in which a light-sensitiveobject, for example a wafer, may be positioned. All of the opticalcomponents of the projection objective according to the invention arecentered around the principal axis HA of the projection objective.

[0034] The projection objective overall has six reflective elements S1,S2, S3, S4, S5, and S6, as well as three refractive components R1, R2,and R3.

[0035] The exact optical data in the Code V format of the example ofembodiment according to FIG. 3 are given in Table 1 below. The opticalsurfaces designated in Table 1 can be found from FIG. 3, viewing fromleft to right in FIG. 3 from the image plane 122 to the object plane120. Surface Radius Thickness Radius Glass 1 −643.3667*    10.000000 45.000000 CaF₂ 2 −128.046493    3.208122  45.000000 Air 3 −111.277787*   8.000000  45.000000 CaF₂ 4 −169.779824   140.540804  53.000000 Air 5−176.803646* −140.540804 P 125.000000 Reflec- tive* 6 −169.779824 P −8.000000 P  53.000000 CaF₂ 7 −111.277787 P*    8.000000 P  45.000000Reflec- tive* 8 −169.779824 P   140.540804 P  53.000000 Air 9−176.803646 P    90.738754 Reference surface Air 10 −691.000000  303.815796 Reference surface Air 11 −453.599666 −303.815796 P397.000000 Reflec- tive 12 −657.027593*  −24.383257 170.000000 Reflec-tive* 13   310.000000   102.409564 Reference surface Air 14   144.555780   8.000000  44.000000 CaF₂ 15   139.463643    5.939134  38.000000 Air16   787.978516*  −5.939134 P  36.000000 A Reflec- tive* 17   139.463643P*  −8.000000 P  38.000000 CaF₂ 18   144.555780 P −102.409564 P 44.000000 Air 19   296.750971*   102.409564 P 140.000000 Reflec- tive*20 −294.000000   117.224195 128.602525 S Air

[0036] Aspherical constants $\begin{matrix}{z = {\frac{({CURV})y^{2}}{1 + \left\{ {1 - \left( {1 + {\left( {{AS}\quad 1} \right)({CURV})^{2}y^{2}}} \right\}^{1/2}} \right.} + {\left( {{AS}\quad 2} \right)y^{4}} + {\left( {{AS}\quad 3} \right)y^{6}} +}} \\{{{({AS4})y^{8}} + {({AD5})y^{10}} + {({AS6})y^{12}} + {({AS7})y^{14}}}}\end{matrix}\quad$

AS 0 AS 1 AS 2 AS 3 AS 4 AS 5 AS 6 AS 7 Surface 1: 0.000000 0.000000  1.896607e−07   5.508863e−11 −5.074820e-14   2.750264e-17 −1.058341e-20  1.845283e-24 Surface: 3 0.000000 0.000000 −2012845e−07 −4.3979980e−11−6.979938e−15 −2.3555673e−18 −1.906964e−22 −2.756038e−26 Surface 5:0.000000 0.000000 −1.661477e−09 −6.249171e−14 −1.362672e−18−1.328683e−22   4.058739e−27 −2.462428e−31 Surface 7: 0.000000 0.000000−2.012845e−07 −4.3979980e−11 −6.979938e−15 −2.3555673e−18 −1.906964e−22−2.756038e−26 Surface 12: 0.000000 0.000000   9.998657e−09 −1.672379e−13  2.197020e−18 −8.214423e−24 −2.720911e−28   3.495656e−33 Surface 15:0.000000 0.000000   6.141170e−08 −1.562833e−11 −4.599874e−15−4.699896e−18 −1.326383e−21 −3.034362e−27 Surface 16: 0.000000 0.000000−5.228692e−08 −5.327721e−12 −3.675170e−15   3.889630e−18   9.903743e−23  3.150874e−26 Surface 17: 0.000000 0.000000   6.141170e−08−1.562833e−11 −4.599874e−15 −4.699896e−18 −1.326383e−21 −3.034362e−27Surface 19: 0.000000 0.000000 −5.026510e−09   1.390563e−13 −1.034164e−17  5.096479e−22 −1.495436e−26   1.826561e−31

[0037] The projection objective according to FIG. 1 has an image-sidenumerical aperture of 0.70 with a reference wavelength of 157.13 nm. Theannular field has a central annular-field radius R of 27 mm at theimage. The overall structural length of the objective is less than 500mm. Structural length in the present Application means the distance ofthe object plane 120 from the image plane 122. The first lens R1 closestto the object plane 120 has a diameter D of the off axis segment of 90mm, the lens R3 closest to the image plane 122 has a diameter D of theoff axis segment of 100 mm, and the largest lens of the system R2 has adiameter D of the off axis segment of 110 mm. It is preferred to useCaF₂ as lens material. Only five of the six reflective components areaspherical. The reflective component farthest from the optical axis HA,the mirror S4 here, is of spherical design. In contrast to this, in theprojection objective according to EP-A-0 779,528 the mirror farthestfrom the optical axis HA is aspherical, which has considerable technicalmanufacturing disadvantages.

[0038] An intermediate image Z is formed in the embodiment shown in FIG.3 between the fourth mirror and the fifth mirror. The physical diaphragmB according to the embodiment shown in FIG. 3 lies on the second mirror.The intermediate image lies in a plane 121 conjugated to the objectplane 120. The first sub-objective or subsystem 1000 comprises the fourmirrors S1, S2, S3, and S4 located in the beam path from the objectplane 120 to the conjugated plane 121 and a single refractive opticalelement, the lens R1. The reduction ratio β₁ of the first sub-objectivethat images the object in the real intermediate image Z is β₁approximately −1. The second sub-objective or subsystem 1002 comprisesthe two mirrors S5 and S6 in the beam path from the conjugated plane 121to the image plane 122 as well as two refractive optical elements, thelenses R2 and R3. The reduction ratio β₂ of the second sub-objectivethat images the intermediate image in the conjugated plane 121 in theimage plane 122, is smaller than 0.25 in magnitude. The overallprojection objective has a reduction ratio of +0.2, i.e. the objectfield 111 in the object plane 120 is imaged in the image plane 122reduced five times in size.

[0039] The invention for the first time describes a projection objectivethat is distinguished by the fact that with a very short structurallength, a projection objective with large image-side numerical aperturecan be described that is simple to manufacture.

What is claimed is:
 1. A projection objective for short wavelengths,preferably of ≦193 nm, especially ≦157 nm, for imaging an object field(111.1) in an object plane (120) into an image field (111.2) in an imageplane (122) with 1.1 a first reflective optical element (S1), a secondreflective optical element (S2), a third reflective optical element(S3), a fourth reflective optical element (S4), a fifth reflectiveoptical element (S5), and a sixth reflective optical element (S6) 1.2 atleast one refractive optical element (R1, R2, R3); 1.3 wherein thereflective and refractive optical elements (S1, S2, S3, S4, R1, R2, R3)have a off axis segment with a diameter D, characterized by the factthat 1.4 the image-side numerical aperture NA≧0.65, and 1.5 that atleast one refractive optical element (R1, R2, R3) has a diameter D ofthe off axis segment that is smaller than ⅓^(rd), preferably smallerthan ¼^(th), and with special preference smaller than ⅕^(th) of thedistance of the object plane (120) from the image plane (122).
 2. Theprojection objective according to claim 1, further characterized by thefact that all of the refractive optical elements (R1, R2, R3) have adiameter D of the off axis segment that is smaller than ⅓^(rd),preferably smaller than ¼^(th), and with special preference smaller than⅕^(th) of the distance of the object plane (120) from the image plane(122).
 3. The projection objective according to claim 1, furthercharacterized by the fact that the distance from the object plane (120)to the image plane (122) is <700 mm.
 4. The projection objectiveaccording to claim 2, further characterized by the fact that thediameter D of the off axis segment of the refractive optical elements is<200 mm.
 5. The projection objective according to claim 2, furthercharacterized by the fact that the diameter D of the off axis segment ofall of the refractive optical elements is <200 mm.
 6. A projectionobjective for short wavelengths, preferably ≦193 nm, especially ≦157 nm,for imaging an object field (111.1) in an object plane (120) into animage field (111.2) in an image plane (122) with 6.1 six reflectiveoptical elements (S1, S2, S3, S4, S5, S6), 6.2 a plane (121) conjugatedto the object plane (120) in which an intermediate image (Z) of theobject field (111.1) is formed, 6.3 a first sub-objective (1000) thatcomprises the optical elements positioned in the beam path from theobject plane (120) to the conjugated plane (121), 6.4 a secondsub-objective (1002) that comprises the optical elements positioned inthe beam path from the conjugated plane (121) to the image plane (122),characterized by the fact that 6.5 the first sub-objective (1000) andthe second sub-objective (1002) each contain no more than threerefractive optical elements.
 7. A projection objective for shortwavelengths, preferably ≦193 nm, especially ≦157 nm, for imaging anobject field (111.1) in an object plane (120) into an image field(111.2) in an image plane (122) with 7.1 six reflective optical elements(S1, S2, S3, S4, S5, S6), 7.2 a plane (121) conjugated to the objectplane (120) in which an intermediate image (Z) of the object field(111.1) is formed, 7.3 a first sub-objective (1000) that comprises theoptical elements positioned in the beam path from the object plane (120)to the conjugated plane (121), with a first reduction ratio β₁, 7.4 asecond sub-objective (1002) that comprises the optical elementspositioned in the beam path from the conjugated plane (121) to the imageplane (122), with a second reduction ratio β₂, characterized by the factthat 7.5 the magnitude of the first reduction ratio β₁ is approximately1, and 7.6 the magnitude of the second reduction ratio |β₂| is <1, andpreferably |β₂|≦0.25.
 8. The projection objective according to one ofclaims 1 to 7, further characterized by the fact that the refractiveoptical elements (R1, R2, R3) are positioned after the second reflectiveoptical element (S2) in the beam path from the object plane (120) to theimage plane (122).
 9. A microlithography projection objective accordingto one of claims 1 to 7, characterized by the fact that three refractiveoptical elements (R1, R2, R3) are provided and two of the threerefractive optical elements (R2, R3) are positioned after the sixthreflective optical element (S6) in the beam path from the object planeto the image plane.
 10. The projection objective according to one ofclaims 1 to 9, further characterized by the fact that at least onesubregion of one lens surface of at least one refractive optical element(R2) is used as a reflective optical element (S5).
 11. The projectionobjective according to one of claims 1 to 10, further characterized bythe fact that the refractive optical elements (R1, R2, R3) comprisealkali-metal and alkaline-earth fluorides, particularly CaF₂, BaF₂, LiF,as lens material.
 12. The projection objective according to one ofclaims 1 to 11, further characterized by the fact that the refractiveoptical elements (R1, R2, R3) are positioned centered on the opticalaxis (HA) of the objective.
 13. The projection objective according toone of claims 1 to 12, further characterized by the fact that no morethan five of the six mirror surfaces of the reflective optical elements(S1, S2, S3, S4, S5, S6) are of aspherical design.
 14. The projectionobjective according to one of claims 1 to 13, further characterized bythe fact that the surface of the reflective optical element (S4) that isfarthest from the optical axis HA of the objective is of sphericaldesign.
 15. The projection objective according to one of claims 1 to 14,further characterized by the fact that the physical diaphragm (B) ispositioned on or near the second reflective optical element (S2). 16.The projection objective according to one of claims 1 to 15, furthercharacterized by the fact that an intermediate image Z is formed afterthe fourth reflective optical element (S4) in a plane (121) conjugatedto the object plane (120) in the beam path from the object plane (120)to the image plane (122).
 17. A projection exposure apparatus with 17.1an illumination system, 17.2 a mask positioned on a first supportsystem, with the mask having a structure and being illuminated by theillumination system with radiation of a wavelength of ≦193 nm, [and]17.3 a light sensitive object positioned on a second support system,characterized by the fact that 17.4 the projection exposure apparatuscomprises a projection objective according to one of claims 1 to 16 forimaging the structure of the mask on the light-sensitive object.
 18. Amethod for producing structural components with a projection exposureapparatus according to claim 15 comprising the following steps: 18.1Positioning a light-sensitive object on the second support system, 18.2Illuminating the structured mask using the illumination system, 18.3Imaging the structure of the mask on the light-sensitive object usingthe projection objective, and 18.4 Exposing the object with radiation ofwavelength of ≦193 nm.